Those circuits are connected with one end of a two-wire telephone line, the other end of which is connected with the telephone exchange controlling several similar lines.
Those internal circuits of a subscriber's set are designed for carrying out several functions, among which:
reception of a ringing signal sent by the telephone exchange,
emission of a signal indicating that the receiver has been hooked off,
emission of the speech signals and reception of the speech signals;
emission of the dialling signals for calling another subscriber identified by those signals.
Those functions must be carried out without defect within operating conditions that may vary a lot from one set to another, especially in relation with the distance between the set and the telephone exchange and in relation with the operating voltage level of the line (the power supply of the set is obtained from the telephone exchange through the line).
Moreover, the various functions must be carried out without errors even under certain abnormal operating conditions, and if they cannot be carried out in those abnormal conditions, the set circuits must anyhow be protected in order not to be destroyed by those abnormal conditions.
One of those abnormal conditions that the set must withstand without being destroyed is the accidental contact of one lead of the electrical power supply network; this network generally provides an electric power at a voltage of 220 volts RMS.
That is the reason why the telephone set internal circuits generally comprise several protection devices against those abnormal operating conditions.
An example of a circuit is shown in FIG. 1.
The circuit comprises two input terminals A and B each one being connected with one of the leads L1, L2 of a two-wire telephone line further connected with two terminals A', B' of a telephone exchange 10.
The circuit of the subscriber's set comprises the following main parts:
a circuit DS for ringing signal detection connected through a capacitor C between terminals A and B and adapted to detect the presence of a ringing signal sent on the line by the telephone exchange; this circuit then controls the operation of an electronic or electromechanic ring tone generator SO. The detection circuit of the ringing signal may be protected by a component (not shown) located between terminals A and B (bidirectional Zener);
after the ringing signal detection circuit DS is located a switch K1 connected with terminal A and a switch K2 connected with terminal B; these switches insulate from the line, when open, the whole set of the circuits except for the DS circuit and capacitor C which remain permanently connected with the line; the switches open when the handset is hooked down, they close when the handset is hooked off;
a rectifying bridge RD has two input terminals E, F and two output terminals G, H; terminal E is connected with switch K1, terminal F with switch K2; terminal G constitutes the common ground of the set circuits, terminal H is both the feeding terminal of those circuits and a terminal through which flow the signals coming from and going to the telephone exchange (speech, dialling, hook signals of the handset);
a circuit or a component CPP for primary protection is connected between terminals E and F for clipping at about 100 RMS volts the voltage received by the rectifying bridge in case of an accidental overvoltage from the line;
one or several integrated circuit(s) CI designed to carry out the various functions of the apparatus (except for the detection of the ringing signal); this (or these) circuit(s) handle the speech signals from a microphone MIC; they emit speech signals on a loudspeaker HP; they emit dialling signals by means of a dial or a digital keyboard CL. Only one circuit CI has been shown;
a stage EHT called "high-voltage stage" is located between the output terminal H of the rectifying bridge RD and an input terminal J of the integrated circuit, this stage being particularly intended to limit the maximal voltage applied to the integrated circuit which could not directly withstand a voltage in the range of 50 to 100 volts which can occur under normal operating conditions at the output of the rectifying bridge RD. The integrated circuit is fed between terminal J and ground G;
finally, a secondary protection circuit CPS is connected between terminal H and ground G; it limits the power used by the high-voltage stage in case of overvoltage, by assigning a limitation of the current used by the high-voltage stage.
One of the main functions of integrated circuit CI (combined with the high-voltage stage EHT which feeds it) is to insure a voltage regulation on terminal J and a regulation of the current which flows from the high-voltage stage towards terminal J. Indeed, the dialling signals as well as the handset hook signal are in fact constituted by the presence or non-presence of a D.C. current higher than a determined threshold on the telephone line. This current is the current that is used by the set and it is essentially the one that flows from the high-voltage stage towards terminal J of the integrated circuit.
Since the current used by the set depends upon the voltage received between terminals E and F and since this voltage depends both upon the feeding voltage of the telephone exchange (which may considerably vary) and the line impedance (which also considerably varies according to the length of the line), the integrated circuit must control both the current and the voltage at point J for maintaining them within a well determined range for each set operating phase (speech reception phase, hook off phase after a call, hook up phase for calling, speech phase). To achieve this purpose, the integrated circuit comprises an output terminal K acting upon the biasing of the high-voltage stage for limiting the used current.
The instant invention particularly relates to what happens during a hook off phase of the handset in case of overvoltage on the line.
In practice, standards are as follows:
1. Calling interruption standard when "called": when the handset is picked up for answering a call, the telephone set starts using a non-null average current. This average current will be detected by the telephone exchange which, at the latest after 150 milliseconds following this detection, will have to stop the ringing signal in order to switch into a speech phase. In case of short lines, the average current will have to be at least equal to 30 milliamperes.
2. Speech phase:
when the speech phase is set, the peak current used (not the average current) must be lower than 60 milliamperes.
3. Hook off operation when "calling"
when the handset is hooked off to call a subscriber, the set starts using an average current in order that the telephone exchange may detect it and make ready to receive a dialling signal; after 400 milliseconds a dialling phase is set and the peak current during this phase must not exceed 70 milliamperes. Finally, after the connecting phase between the caller and the called person, the talking phase is again existing during which the peak current must not exceed 60 milliamperes.
As a general rule, current limitations are required to meet these standards, but moreover, limitations are useful to avoid an excessively high power consumption when overvoltages are involved.
As it is necessary in certain cases (short lines) to use an average current of at least 30 milliamperes when hooking off the handset after a ringing signal, it appears that a peak current limitation cannot be assigned below 110 milliamperes.
Thus, the integrated circuit CI has to set a peak current limitation to 110 or 120 milliamperes but not below this threshold for about 400 milliseconds after the hooking off of the handset (that is after the voltage is applied to the integrated circuit CI). In this manner, the calling interruption standards when "called" can be complied with. Then, after 400 milliseconds, the integrated circuit will limit the current to 60 mA, either for the dialling phase, or for the speech phase.
If an overvoltage occurs on the line during the handset hook off phase, this overvoltage will be limited to about 100 volts RMS by the primary protection component CPP, and it will give rise to a power consumption of about 6 watts for about 400 milliseconds, and of about 3 watts afterwards.
Those power requirements are too high and demand the use of expensive transistors in the high-voltage stage EHT.
A proposal has already been made to limit the supplementary peak power consumption after a period higher than 400 milliseconds; this limitation is achieved by means of the secondary protection circuit CPS shown in FIG. 1.
This circuit is carried out by means of components external to the integrated circuit CI (Zener diode, resistors, capacitor, transistor); it acts upon the biasing of the high-voltage stage for limiting to about 30 mA the used peak current; this circuit acts only in case of overvoltages and after a certain delay further to the application of an overvoltage.
The delay is broadly calculated so as to be much higher than 400 milliseconds in spite of the scattering of the resistor and capacitor values of circuit CPS. In this way, the protection circuit does not impair the hooking off phase of the receiver when "called". In practice, one has to provide for an average delay of several seconds for taking this scattering of the components into account.
Consequently, 3 watts may be scattered for several seconds, after a consumption of 6 watts for 400 milliseconds.